1. Field of the Invention
The present invention relates to an ion source used in an ion implanter, and in particular, a mass separation filter for extracting ions with a desired mass provided in the ion source.
2. Related Art
An ion source generates plasma from a gas introduced into a vacuum vessel to extract as an ion beam. It is used in areas such as the introduction of impurities to semiconductors, TFT for liquid crystal, solar cells, and the like, processing through etching and sputter by an ion beam, and furthermore, in deposition and reforming by ions.
In particular, large area ion beams are greatly used in the ion implantation of semiconductors and reforming of materials, obtaining high productivity when producing products such as flat panels on a large scale.
The ion beam for a semiconductor wafer in general ion implantation is smaller than the above ion beams. This ion beam implants only one ionic species undergoing mass spectrometry onto a substrate thereof. To use a large area ion beam in this preferred method, it is necessary to increase scale across the board, but increasing the size of the device is difficult. In addition, large sector dipole magnets used for the wafer would become costly.
As a related art, there is a mass separation device disclosed in Japanese Patent Journal No. 2920847. This device, as shown in FIG. 7, includes an incidence plate 31 having a plurality of through holes 30 with mutually parallel axis lines, and an ion incidence plate 33 which is arranged in parallel with the incidence plate and has a plurality of through holes 32 with axis lines at a predetermined angle xcex8 with respect to the axis line of the through holes of the incidence plate 31. This device is also provided with field generating means B that generates a magnetic field perpendicular to the axis lines of the respective through holes.
With this mass separation device, it is possible to simultaneously separate mass across a wide area since mass separation is performed through only a difference in the curving angle of the ions. However, in this device, the direction of ions incident to the implantation incidence plate differs the direction of the ions emitted from the ion incidence plate, therefore, the incidence direction and the emitting direction of the ion beam passing via an extraction electrode cannot be made uniform, and it is difficult to arrange the plasma electrode, extraction electrode, acceleration electrode, and ground electrode in parallel at the bottom portion of the plasma chamber to extract ions of a desired mass in a fixed direction.
Also, a mass separation system from Aitken is disclosed in the specification of European Patent No. 1090411. In this system, two dipole magnets sequentially placed along a beam axis form a quadrupole type lens. The two magnets are oriented such that their magnetic fields are not parallel and face in opposite directions perpendicular to the beam axis. This quadrupole lens forms a linear ion beam extracted from a slit in the plasma electrode, and ions linearly converge at the exit portion of the lens thereof.
Therefore, since this focus position changes according to ionic mass, mass selection becomes possible and ions with a necessary mass can be separated. However, this device requires a large space, and the beam has a long trajectory direction. The mass separation filter should prevent the beam from impinging on the internal portion of the filter and must collimate, therefore maintaining the beam in parallel is difficult. Accordingly, the space of the ribbon ion beam must be widened, and the horizontal space of the mass separation filter needs to be made larger.
Further, a mass separation filter 40 using a Wien filter that separates mass through the action of electric and magnetic fields is disclosed in Japanese Patent Laid-Open Publication No. 5-82083 (corresponding U.S. Pat. No. 5,189,303 specification). This device, as shown in FIG. 8A, has a plasma electrode 41, an extraction electrode 42, an acceleration electrode 44, and a ground electrode 45 arranged on an ion source exit side. The extraction electrode 42 at which ionic speed is at a low stage is constituted by an extraction electrode 42a and a mass separation electrode 43. A Wien filter 50 is respectively provided on each through hole 52 of the extraction electrode 42a. 
Expanding a portion of the extraction electrode 42a, as evident in detailed views of FIG. 8, a vertical section in FIG. 8B and a horizontal section in FIG. 8C, it includes magnets 48 disposed facing each other on segmented electrode plates 46, and structures a Wien filter generating an electric field E in direction x, and a magnetic field B in direction y. Further, a mass separation electrode 43 with little voltage conforming to a position of a through hole is provided immediately rearward of the extraction electrode 42a, thereby enabling the mass separation of a large area ion beam. In this case, ions of a desired mass pass through holes without change, while ions with undesired masses do not pass through the through holes. In other words, ions with excessively large or small masses are eliminated, therefore resolution is high and size reduction is possible.
However, the Wien filter adds an electric field applied parallel to the beam direction to accelerate ions, and furthermore, requires an electric field perpendicular to the beam direction which generates a filtering effect through the electric field and the magnetic field. In addition, much of that plate/electrode area necessitates structures for generating the crossed electric fields and magnetic field, which limit the electrode release area relating to beam transport, therefore in addition to restricting total beam current, it is difficult to obtain satisfactory homogeneity.
In view of the foregoing situation, it is an object of the present invention to provide a mass separation filter and a mass separation method thereof, as well as an ion source using the same, in order to generate a large area ion beam from ions with a desired mass, allowing the selective rejection of unnecessary ions, in addition to simplifying and reducing the size of the electrode structure of the ion source.
In order to achieve the above-mentioned object, the present invention has a structure as described in the claims. The mass separation filter of the present invention is characterized by having a first magnet forming a first magnetic field in a direction orthogonal to a beam axis of an ion beam; a second magnet provided in series to the first magnet along the beam axis, forming a second magnetic field which is orthogonal to the beam axis and in parallel with and opposite the first magnetic field; and a collimator wall for forming a beam channel having a first and a second curved channels formed within the first and second magnetic fields such that selected ions of a desired mass can pass from the first curved channel slanted by the first magnetic field to the second curved channel which is slanted in a direction the reverse of the first magnetic field by the second magnetic field.
According to this structure, it is possible to extract ions of a desired mass from ions entering the mass separation filter and passing through a beam channel with a channel that is inversely curved by the magnetic fields of the first and second magnets, in addition to allowing the directions in which ions enter and are emitted to be identical to the direction of the beam axis.
Also, according to the first aspect of the present invention, a large area ion beam of the present invention includes a plasma chamber; means for introducing gas with a controlled flow into the plasma chamber; an energy source for ionizing the gas within the plasma chamber; a plasma electrode that forms a plasma chamber wall with an oblong opening, and extracts positive ions from the opening; an extraction electrode for setting a controllable value of the kinetic energy of the ions, and provided parallel to and with a low potential with respect to the plasma electrode in order to extract ions passing the plasma electrode; and a mass separation filter provided rearward of the plasma electrode and having a plurality of openings aligned with the extraction electrode in order to select a desired mass or a range of mass.
According to this structure, it is possible to selectively eliminate unnecessary ions, while allowing ions of a desired mass to pass along the collimator wall through the action of magnetic fields of the first and second magnets within the mass separation filter, without changing the arrangement of the electrode structure of the ion source. In addition, the structure of the mass separation filter may be formed by the collimator wall and the first and second magnets, therefore, the structure is simple. Also, control for extracting ions of a desired mass is easy because no effects are generated by the interaction of the magnetic fields and electric field since the incident ions are deflected by only the magnetic fields. Further, a beam channel is realized which curves in a shape that inverts a channel curved in one direction, therefore, it has excellent ion convergence, and enables a reduction in the size of the mass separation filter used in a large area ion beam passing through slits with a high aspect ratio.
According to a preferred embodiment of the present invention, the first and second magnets are permanent magnets, and are mounted in a metal tube through which coolant flows. In addition, the beam channel formed by the collimator wall is a substantial S-shape and not parallel to the magnetic fields. Further, the collimator wall is created from a thin metal plate or graphite, and has at least a pair of curved walls and a pair of side walls provided facing each other in order to form the first and second curved channels. In the case of graphite collimator walls, it is possible to machine solid graphite, or manufacture it from soft graphite sheets.
In addition, according to another structure of the present invention, the beam trajectory slanted by the first and second magnetic fields is structured to shift an emission opening position of the beam towards the mass separation filter with respect to the incidence opening position of the beam. The two opening positions allow the passing of the forward traveling beam, therefore, by overlapping them when viewed from the axial direction of the ion beam, unnecessary ions, electrons, and the like can be reliably separated from the ion beam.
Also, when overlapping the two opening positions, it is possible to increase the total ion beam amount passing through since the forward traveling beam renders a small opening shift amount with direct emission.
Therefore, by forming the first magnetic field orthogonal to the beam axis of the ion beam, and forming the first and second magnetic fields mutually opposite and parallel, as well as orthogonal to the beam axis, it is possible in the present invention to render the traveling direction of incident ions and emitted ions identical to the direction of the beam axis, allowing easy alignment of each electrode of the ion source. Also, in forming a curved beam channel by collimator walls structured from a curved wall and a side wall, it is possible to eliminate unnecessary ions by only allowing ions of a desired mass to pass along the collimator walls. Furthermore, unnecessary ions, electrons and the like are separated from the ion beam by adjusting the shift amount between the incidence opening position and the emission opening position of the beam channel of the ion beam, thereby allowing an increase in the total ion beam amount passing through.
Further, the mass separation method according to the present invention includes the steps of forming a first magnetic field orthogonal to a beam axis of an ion beam or forming mutually opposing and parallel first and second magnetic fields orthogonal to the beam axis; deflecting an ion beam within the magnetic fields along a curved channel formed by a collimator wall created from at least a pair curved walls and a pair of side walls provided facing each other; and passing selected ions of a desired mass while colliding forward traveling ions and unnecessary ions into the collimator wall; therefore ions with a desired mass can be selected by the curved beam channel with a simple magnet structure. In addition, ionic convergence is excellent, and it is possible to perform mass separation of a large area ion beam passing through slits with a high aspect ratio.
In addition, the structure of the mass separation filter is formed by the first and second magnets and the collimator walls, therefore, it has a simple structure. Since the incident ions are deflected by only the magnetic fields, designing a collimator that does not generate effects from the interaction of the magnetic and electric fields is easy. Further, according to the present invention, a beam channel is realized which curves in a shape that inverts a channel curved in one direction, therefore, it has excellent ion convergence, and enables a reduction in the size of the mass separation filter used in a large area ion beam passing through slits with a high aspect ratio.